Fix parallel tool call results lost during session chain reconstruction

src/claude_agent_sdk/_internal/sessions.py

@@ -833,7 +833,40 @@ def _pick_best(candidates: list[_TranscriptEntry]) -> _TranscriptEntry:
         chain_cur = by_uuid.get(parent) if parent else None
 
     chain.reverse()
-    return chain
+
+    # Parallel tool calls produce multiple user messages (tool results) that
+    # share the same parentUuid.  The linear walk above follows only one path
+    # from leaf to root, so sibling tool-result messages are missed.  This
+    # pass finds and inserts those missing siblings in file order.
+    children_by_parent: dict[str, list[_TranscriptEntry]] = {}
+    for entry in entries:
+        p = entry.get("parentUuid")
+        if p:
+            children_by_parent.setdefault(p, []).append(entry)
+
+    chain_uuids: set[str] = {e["uuid"] for e in chain}
+
+    expanded: list[_TranscriptEntry] = []
+    for entry in chain:
+        p = entry.get("parentUuid")
+        if p:
+            for sib in children_by_parent.get(p, []):
+                sib_uuid = sib["uuid"]
+                # Only include user-type siblings (parallel tool results).
+                # Assistant-type siblings represent conversation branches
+                # (e.g., retries) and should not be merged.
+                if (
+                    sib_uuid not in chain_uuids
+                    and sib.get("type") == "user"
+                    and not sib.get("isSidechain")
+                    and not sib.get("teamName")
+                    and not sib.get("isMeta")
+                ):
+                    chain_uuids.add(sib_uuid)
+                    expanded.append(sib)
+        expanded.append(entry)
+
+    return expanded
 
 
 def _is_visible_message(entry: _TranscriptEntry) -> bool:

tests/test_sessions.py

@@ -1062,6 +1062,57 @@ def test_only_progress_entries_returns_empty(self):
         result = _build_conversation_chain(entries)
         assert result == []
 
+    def test_parallel_tool_calls_includes_siblings(self):
+        """Parallel tool results sharing the same parentUuid are all included."""
+        entries = [
+            {"type": "user", "uuid": "U1", "parentUuid": None},
+            {"type": "assistant", "uuid": "A1", "parentUuid": "U1"},
+            {"type": "user", "uuid": "U2", "parentUuid": "A1"},  # tool_result 1
+            {"type": "user", "uuid": "U3", "parentUuid": "A1"},  # tool_result 2
+            {"type": "assistant", "uuid": "A2", "parentUuid": "U3"},
+        ]
+        result = _build_conversation_chain(entries)
+        uuids = [e["uuid"] for e in result]
+        assert uuids == ["U1", "A1", "U2", "U3", "A2"]
+
+    def test_parallel_tool_calls_three_siblings(self):
+        """Three parallel tool results are all included in file order."""
+        entries = [
+            {"type": "user", "uuid": "U1", "parentUuid": None},
+            {"type": "assistant", "uuid": "A1", "parentUuid": "U1"},
+            {"type": "user", "uuid": "U2", "parentUuid": "A1"},  # tool_result 1
+            {"type": "user", "uuid": "U3", "parentUuid": "A1"},  # tool_result 2
+            {"type": "user", "uuid": "U4", "parentUuid": "A1"},  # tool_result 3
+            {"type": "assistant", "uuid": "A2", "parentUuid": "U4"},
+        ]
+        result = _build_conversation_chain(entries)
+        uuids = [e["uuid"] for e in result]
+        assert uuids == ["U1", "A1", "U2", "U3", "U4", "A2"]
+
+    def test_parallel_siblings_excludes_sidechain(self):
+        """Sidechain siblings should not be pulled into the main chain."""
+        entries = [
+            {"type": "user", "uuid": "U1", "parentUuid": None},
+            {"type": "assistant", "uuid": "A1", "parentUuid": "U1"},
+            {"type": "user", "uuid": "U2", "parentUuid": "A1", "isSidechain": True},
+            {"type": "user", "uuid": "U3", "parentUuid": "A1"},
+            {"type": "assistant", "uuid": "A2", "parentUuid": "U3"},
+        ]
+        result = _build_conversation_chain(entries)
+        uuids = [e["uuid"] for e in result]
+        assert uuids == ["U1", "A1", "U3", "A2"]
+
+    def test_linear_chain_unchanged_with_sibling_fix(self):
+        """Linear chains (no parallel calls) produce identical results."""
+        entries = [
+            {"type": "user", "uuid": "a", "parentUuid": None},
+            {"type": "assistant", "uuid": "b", "parentUuid": "a"},
+            {"type": "user", "uuid": "c", "parentUuid": "b"},
+            {"type": "assistant", "uuid": "d", "parentUuid": "c"},
+        ]
+        result = _build_conversation_chain(entries)
+        assert [e["uuid"] for e in result] == ["a", "b", "c", "d"]
+
 
 class TestSessionMessageType:
     """Tests for the SessionMessage dataclass."""